cDNA cloning and tissue expression of plasma lysozyme in the eastern oyster, Crassostrea virginica

The cDNA sequence of a 17,861 Da lysozyme first purified from plasma of eastern oysters ( Crassostrea virginica) was identified and its complete amino acid sequence deduced. The amino acid sequence of the plasma lysozyme, designated cv-lysozyme 1, contained both a unique and a conserved region when compared to the amino acid sequences of other bivalve lysozymes. In situ hybridisation located cv-lysozyme 1 gene expression in mantle and gill cells in standard histological sections. Quantitative real-time RT-PCR detected cv-lysozyme 1 expression in all organs examined and circulating haemocytes. The number of cv-lysozyme 1 mRNA transcripts was particularly high in mantles and labial palps suggesting those organs are the main sites of cv-lysozyme 1 synthesis. Cv-lysozyme 1 enzyme activity measured by lysing Micrococcus lysodeikticus bacteria and expressed in units per gram tissue was highest in mantles, labial palps and gills. Most cv-lysozyme 1 enzyme activity in oysters was found in plasma. Cv-lysozyme 1 main organs of synthesis, its abundance in plasma and its strong antimicrobial properties suggest its main role is in oyster host defences.